Viessmann Vitodens 100-W Compact boiler and pump overrun

[MJN]

I am currently the lucky owner of an Ideal Icos connected to a sealed S-plan system with an unvented HW cylinder. Whilst it has never really given us any trouble I am acutely aware that by all accounts I am actually playing a slow-motion game of Russian roulette and it is only a matter of time to learn the irony of the manufacturer's name...

So, I've decided to research now what boiler to ultimately replace it with as I can see the benefit of doing so whilst not under duress. To that end I'm currently settling on the Viessmann 100-W Compact as by many accounts it (the newer WB1B model?) seems a fairly acceptable boiler. My personal interest stems from its simplicity by virtue of being a heat-only boiler with fewer internal parts to go wrong, it being as close to a drop-in replacement as one could hope for, support for weather compensation, and reasonably low minimum modulation output.

Looking at the installation instructions, which are by far and away the most scant that I have come across, it would appear that there is no requirement for pump overrun given that the pump connection seems entirely optional (pg 13 refers). Can anyone confirm that this is correct? Does it achieve its heat dissipation during overrun using the fan (as per the Icos) or does it simply not have such a dissipation requirement in the first place?

Any other comments from 'those in the know' on my current choice would be most welcome.

Mathew

 

[JohnD]

I have one (a WB1A) and I phoned the manufacturers to check if it needed a bypass.

It doesn't need anything extra, as the pump continues running for about 10 minutes after the boiler stops firing, sending it to the rads or boiler, depending what it last fed.

I too chose it for its simplicity, and its long warranty, and its stainless heat exchanger.

As the exchanger is stainless tube, it does not hold a lot of heat like the old iron boilers did.

I don't think "the pump connection seems entirely optional" is a correct interpretation.

 

[MJN]

It doesn't need anything extra, as the pump continues running for about 10 minutes after the boiler stops firing, sending it to the rads or boiler, depending what it last fed.

Only if connected to the boiler of course, but even then not with an S-plan system because when there is no call for heat the heating and HW circuits are closed leaving only the bypass available. (I'm assuming also when you said 'boiler' you meant cylinder?)

As the exchanger is stainless tube, it does not hold a lot of heat like the old iron boilers did.

I guess that is key to lack of a heat dissipation requirement.

I don't think "the pump connection seems entirely optional" is a correct interpretation.

You might be right, but I'm not sure sure how I can read the instructions any other way!

Mathew

 

[JohnD]

Only if connected to the boiler of course

only if what is connected to the boiler?

but even then not with an S-plan system because when there is no call for heat the heating and HW circuits are closed leaving only the bypass available

the stats are connected to the boiler, and the boiler turns the pump on and controls the valves

I don't think "the pump connection seems entirely optional" is a correct interpretation.

You might be right, but I'm not sure sure how I can read the instructions any other way!

please type the sentence(s) that you think say that

 

[MJN]

only if what is connected to the boiler?

Sorry, the pump.

the stats are connected to the boiler, and the boiler turns the pump on and controls the valves

Not in my system. The room stat is connected to the CH zone valve which in turn is connected to the pump and boiler demand input. Similarly for the HW timer (and HW valve). I don't know there even were boilers that controlled the external valves themselves (this boiler certainly isn't one).

please type the sentence(s) that you think say that

From pg 13: 'Pump can be connected externally of the boiler through the external terminal box'

And the diagram above it doesn't show any pump wiring. Furthermore, nowhere in the manual does it detail how the pump is connected (i.e. which pins on connector 20 is L, N and Earth).

Mathew

 

[JohnD]

only if what is connected to the boiler?

Sorry, the pump.

The pump always will be connected to the boiler.

the stats are connected to the boiler, and the boiler turns the pump on and controls the valves

Not in my system. The room stat is connected to the CH zone valve which in turn is connected to the pump and boiler demand input. Similarly for the HW timer (and HW valve).

the old system is not the same.

please type the sentence(s) that you think say that

From pg 13: 'Pump can be connected externally of the boiler through the external terminal box'

but it doesn't say "or can be left unconnected?"

And the diagram above it doesn't show any pump wiring. Furthermore, nowhere in the manual does it detail how the pump is connected (e.g. which pins for L, N and Earth).

Item 20, page 40? X1 is neutral, so 2 must be. X18 and X19 are earth. Possibly there is a plug-on multiconnector.

I can't see what K1 does.

Anyway, you'll be getting it installed by a Viessmann approved installer.

 

[transam]

Veissmann heat only ( compact ) does not require a pump over run !

There is I beleive an inbuilt frost stat facility on this boiler , & for that to work u would need to wire the pump back to the boiler , other than that the boiler does not require pump over run !

 

[MJN]

Thanks Transam. Looking at the installation manual for John's WB1A it seems fundamentally different from the WB1B with regards to external connectivity so it is no wonder we seem to coming at things from completely different cross purposes!

Regarding frost protection, the boiler is located within the kitchen and I would have the frost protection facility of the CM927 programmer activated so I am assuming I should be okay.

Mathew

 

[JohnD]

Veissmann heat only ( compact ) does not require a pump over run !

Mine (WB1A) does it though. Every time the boiler stops firing the pump runs for a further 10 mins or so. I phoned the makers and asked, they said it does this through whatever circuit last called for heat, so it does not need a bypass.

Thanks Transam. Looking at the installation manual for John's WB1A it seems fundamentally different from the WB1B with regards to external connectivity so it is no wonder we seem to coming at things from completely different cross purposes!

Sorry!

 

[MJN]

Thanks Transam. Looking at the installation manual for John's WB1A it seems fundamentally different from the WB1B with regards to external connectivity so it is no wonder we seem to coming at things from completely different cross purposes!

Sorry!

No worries - it at least got me thinking!

Mathew

 

[mysteryman]

Pump overrun is optional on the WB1B Compact.

 

[Agile]

The manufacturers do not insist on using a pump over run connection.

I expect that is to enable it to be connected just like an old boiler which was when powered only when heat was required.

However its far better to let it control the pump and be activated by a call for heat. That reduces thermal stress and enables the frost protection to best operate.

Tony

 

[MJN]

The manufacturers do not insist on using a pump over run connection.

I expect that is to enable it to be connected just like an old boiler which was when powered only when heat was required.

Thanks Tony. Perhaps during upcoming kitchen renovations I will run a cable from boiler to the airing cupboard to at least give the option of pump control when the time comes, particularly should I end up changing my choice of boiler.

That said, my current S-plan system has two radiators (lounge where the stat is, and a bathroom towel radiator) with no TRVs. The auto-bypass is redundant and hence is wound right back to reduce inefficiencies.

If I am to allow the boiler to overrun the pump would you recommend utilising the auto-bypass or converting the sytem to Y-plan to allow the useful dumping of heat without the drawbacks of a bypass?

However its far better to let it control the pump and be activated by a call for heat. That reduces thermal stress and enables the frost protection to best operate.

Fully understand on the frost protection front, but could you elaborate on the thermal stress elements? As John touched on the heat exchanger in this boiler has a very low thermal mass and hence there will be minimal post-burn heating within the boiler, particular given the fan purge cycle. Why would there be thermal stress letting it cool naturally?

Mathew

 

[mysteryman]

It is best to use a W Plan valve and equip the boiler with weather compensation [and remove the bypass]. There is a specific kit for this.

The heat exchanger is not low water content or lightweight; it is the best engineered one on the market.

 

[MJN]

It is best to use a W Plan valve and equip the boiler with weather compensation [and remove the bypass]. There is a specific kit for this

Why W plan over Y plan?

My house (built in 2007) has relatively low heating requirements and so I can currently benefit from supplying the CH and HW at the same time. Would a W plan provide alternative benefits that would outweigh this current modus operandi?

Mathew